Halloysite tubule nanoreactors in industrial and agricultural applications
Halloysite is a two-layered aluminosilicate which has a predominantly hollow tubular structure in the submicron range and is chemically similar to kaolinite . It is mined commercially from natural deposits in USA, New Zealand, China, Turkey, and Malaysia . Dragon mine in Utah has exceptionally pure halloysite with nanotubes exceeding 99 %. These minerals are formed from kaolinite over millions of years due to the hydrothermal processes . Layers are rolled into tubes due to the strain caused by lattice mismatch between adjacent silicone dioxide and aluminum oxide sheets [1-3].
Halloysite was discovered in 18th century and named in honor of Omalius d’Halloy who analyzed the mineral . In early years it was very difficult to distinguish halloysite from other minerals, particularly from kaolinite. However, X-ray analysis has shown that it has unique crystalline structure . Halloysite has been extensively utilized as a raw material for ceramics industry, especially for the manufacture of porcelain, and bone china [1,7]. Having nanotubular structure, halloysite particles can potentially be applied in several fields of nanotechnology. These multilayer tubes are commonly used for plastic composites, in controlled release applications [8-15], and may be coated with metal by electroless plating to make conductive fillers . Due to their porous structure and high catalytic activity, halloysite particles can be used in remediation of acid mine drainage, petroleum conversion in refining industries, as well as in the separation of liquids and gaseous mixtures. This paper summarizes the structure, physicochemical properties and major application areas of halloysite nanotubes.